Some heart patients experience episodes of atrial tachyarrhythmia, including atrial fibrillation (AF). Although AF episodes may not be immediately life-threatening, AF episodes may be associated with extreme symptoms, a reduced quality of life, and a reduced cardiac output.
For heart patients having a multi-chamber pacemaker, AF episodes present an additional problem, in that the pacemaker may coordinate ventricular pacing with atrial activity. When an AF episode begins, it is undesirable for ventricular pacing to be timed according to atrial activity. Accordingly, some pacemakers are equipped with “mode switching” capability. The principal purpose of mode switching is to prevent the pacing system from delivering ventricular paces that track atrial activity when the atrium experiences an episode of atrial tachycardia. When the atrial rate is normal, the pacemaker assumes a tracking mode, such as DDD/DDDR, in which ventricular pacing tracks atrial activity. When an AF episode occurs, however, the pacemaker switches to a non-tracking mode, such as DDIR, and paces the ventricle independently of atrial activity.
When an AF episode occurs, the pacemaker or another medical device may apply therapy to terminate the AF episode. Therapy may comprise application of a shock or a drug. Another therapy for atrial tachycardia is overdrive pacing, in which the pacemaker paces one or both atria at a rate faster than their intrinsic rhythm. Overdrive pacing is often effective in disrupting circus arrhythmia and terminating an AF episode. An AF episode may also terminate spontaneously.
It has been observed that a second AF episode may occur within seconds or minutes after the termination of the first AF episode. Although a recurrent AF episode does not always follow the termination of a preceding AF episode, it has been demonstrated clinically that a patient may have an increased risk of a recurrent AF episode shortly after an AF episode has terminated. The phenomenon is called recurrent atrial fibrillation.